One of conventional fuel cell single cells is described in Patent Document 1 titled “Fuel Cell and Electrode Structure for Fuel Cell”. The fuel cell described in Patent Document 1 has an MPL that is composed of a sparse area made of electrically conductive particles with a large particle size and a dense area made of electrically conductive particles with a smaller particle size. The upper face of the MPL is in contact with a gas diffusion layer, and the lower face is in contact with a catalyst layer.
In the fuel cell, the particles of the dense area are of such a particle size that the saturated vapor pressure in the pores determined by the Kelvin equation is higher than the saturated vapor pressure in an open space. In the fuel cell, this makes the saturated vapor pressure in the sparse area lower than the saturated vapor pressure in the dense area, and condensation of water vapor in the dense area is thereby reduced, which is generated in the catalyst layer.